It is generally known that injuries may arise if the moving parts of the body are subjected to high stress during walking or running. Such injuries arise in particular with sports requiring a great amount of movement. This applies for both contact sports, for example soccer, and non-contact sports, for example running or jogging.
In particular, long distance runners are often concerned with injuries or stress on the moving parts of the body, which may reduce performance and may lead to the inability to further perform this sport. The most serious of these injuries are injuries to the knee or injuries to the cartilage. Such injuries arise often due to the turning movement that accompanies the gait cycle when the foot contacts the ground and pushes off from the ground. Two types of turning movements that are considered as being dangerous are pronation and supination. Both lead often to injuries of the knee. Pronation is a rotation or turning of the foot from a lateral side, i.e., the outer side, of the foot to a medial side, i.e., the inner side, of the foot. During the gait cycle, the foot typically contacts the ground at first with the outer part of the heel. The weight is then shifted to the forefoot part to start the pushing-off phase. During this pushing-off phase, the pronation turning to the medial side of the foot starts. Supination is a corresponding turning of the foot from the medial side to the lateral side of the foot. Both often lead to injuries of the knee.
To address the above-mentioned problems, several approaches have been suggested in the prior art. For example, U.S. Pat. No. 4,615,126, the disclosure of which is hereby incorporated herein by reference in its entirety, describes a sole unit, in particular for sports shoes, with segments in the forefoot part at the third to fifth metatarsal-phalangeal joints of the foot with an increased flexibility in comparison to the remaining part of the sole unit. Such a configuration may reduce the resistance of the sole unit with respect to the bending of the foot along the third to fifth metatarsal-phalangeal joints, whereby tensions in the respective muscles are reduced and a pushing-off movement in particular by the first and second metatarsal-phalangeal joints of the foot is achieved. The provision of the more flexible part in the forefoot part may lead to a reduction of the pronation or supination movement.
Another known solution, as shown for example in U.S. Pat. No. 5,025,573, the disclosure of which is hereby incorporated herein by reference in its entirety, relates to a sole unit with different parts made out of substances with different elastic properties. The described sole unit consists of two layers, i.e., a lower layer of a firm material (which may also serve as the outsole) and an upper softer layer. The lower layer has an increased thickness in the heel part and in the part of the arch that serves as an upper stabilizing surface for the foot. The superposed softer damping layer is formed complementary to the lower firmer layer and to the foot, where its thickness varies in dependence of the transverse position.
Yet another known solution is, for example, described in International Patent Application No. WO95/03719 filed under the PCT, the disclosure of which is hereby incorporated herein by reference in its entirety. According to this approach, pronation is counteracted by inserting into the heel part of the sole unit a wedge-like element being on its lower side in contact with the outsole. The wedge-like element comprises upper and lower layers of a composite material of carbon fibers with a springy core material in between. The wedge-like element tapers from the medial side to the lateral side and from the heel to the midfoot area. In other words, the wedge-like element tapers in two directions.
In addition, U.S. Pat. No. 4,642,911, the disclosure of which is hereby incorporated herein by reference in its entirety, discloses a sole construction that is to be further described with reference to FIG. 12 of the present application. In particular, the '911 patent discloses a sole unit that is suggested to control pronation consisting of a material with a dual density. To this end, the sole unit is made out of two different materials A and B, where material A has a higher density compared to that of material B. FIG. 12 depicts in cross-section such a sole unit, including two wedge-like elements 150, 155 that may be glued or melted to each other. The '911 patent describes that the formation of the wedge-like element 150 as a material of higher density causes a continuous decrease of the density of the sole unit from the medial side to the lateral side, and that thereby a support of the medial side of the foot is achieved. The '911 patent describes further that for a control of supination movement the wedge 150 with the material of higher density may be arranged on the lateral side of the sole unit.
In the following, a manufacturing method for sole units according to the prior art is discussed, consisting of parts with different density. Reference is made thereto to FIGS. 11A-11C of the present application. As shown in FIG. 11A, a tool 100 is filled with a polymer material 120 to be formed having a certain first density. The block of material 120 may include in a desired part a second material of higher density 125. In FIG. 11A, this second material 125 is shown as being embedded into the first material 120. Alternatively, it is also possible to position two different blocks 120, 125 separately into the tool. Finally, the tool 100 is closed by positioning a second half of the tool 100′ onto the first half of the tool (FIG. 11B).
Subsequently, the sole unit is formed into its desired shape (FIG. 11C) according to the tool from the material blocks 120, 125 (i.e., preforms) by the application of pressure and heat. The above mentioned techniques, however, have a severe disadvantage that for achieving the desired density variation over a transverse and/or longitudinal area of the sole unit, two different materials of different densities are at first formed and subsequently connected to each other. This requires not only increased labor consumption, but also leads to interconnection problems, since the two materials with different densities have to be compatible with each other. Furthermore, it has to be taken into account that shoes, in particular sport shoes, are mass-produced products that are produced by machines.
For at least these reasons, the complex forming of two materials to be interconnected for achieving a desired density variation is only possible with a high expenditure.